The technique as such for making separate joints of this type is well known in the art. A suitable tool comprises generally two separate tool-parts which co-operate for producing said joint. A first tool-part has the form of a punch which in an essentially linear stroke is driven in the direction of a co-axial second tool-part in the form of a matrix with a matrix cavity at the bottom of which an anvil is arranged.
For making the joint the sheet-formed members are positioned against the second tool-part, the matrix, provided with movable matrix elements, in certain embodiments arranged sliding laterally on a support surface against the forces from a spring element. The spring element is generally constituted by a ring made of an elastomer or a toroid formed metal spring surrounding the movable matrix elements.
The approaching punch impacts on the surface of one of the members to be joined. The material of the two members is first drawn into the matrix cavity and subsequently due to the interaction between the punch and the anvil at the bottom of the cavity laterally extruded, thereby displacing the movable matrix elements outwardly creating in the sheet-formed members a mushroom formed button which interlocks the members. The dimension of known matrixes in the direction of the essentially linear stroke or movement of the co-operating punch, i.e. perpendicular to the plane of the sheet-formed members to be joined is typically many times the diameter of the matrix cavity.